Tunable interplay between3dand4felectrons in Co-doped iron pnictides

Abstract

We study the interplay of 3d and 4f electrons in the iron pnictides CeFe1−xCoxAsO and GdFe1−yCoyAsO, which correspond to two very different cases of 4f -magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe 3d electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: 0.05 < x < 0.2 for CeFe1−xCoxAsO and 0.05 < y < 0.25 for GdFe1−yCoyAsO, showing a maximum transition temperature Tsc of about 13.5 K for Ce and 19 K for Gd. In both compounds, the 4f -electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co 3d electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches T Co C ≈ 75 K at x = 1 and y = 1. In the Ce-compounds, the Néel temperature T Ce N increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to x ≃ 0.8 (T Ce N ≈ 4 K). Furthermore, evidence of Co-induced polarization on Ce-moments is observed on the Co-rich side. In the Gd-compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For 0.7 ≤ y < 1.0, the system undergoes a possible magnetic reorientation below the Néel temperature of Gd (T Gd N ). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the 3d-4f electrons give rise to a rich phase diagram in the rare-earth iron pnictides. PACS number(s): 74.70. Xa,71.20.Eh,75.20.Hr

Magnetocrystalline anisotropic effect inGdCo1−xFexAsO(x=0,0.05)

“…In the Ce(Fe,Co)AsO series, superconductivity appears while the magnetic order at T Fe N is quickly suppressed upon substituting Fe with Co. 14,15 The Ce Néel temperature slightly increases near the Fe-3d magnetic instability, but shows a nearly unchanged value on further increasing the Co concentration, 14 indicating a strong coupling between Ce 4f electrons and Fe 3d electrons in CeFeAsO, as previously reported by neutron scattering and muon spin relaxation measurements. 16,17 On the Co-rich side, the ferromagnetically ordered Co ions have a strong polarization effect on the AFM order of Ce moments.…”

“…14,19 For the case of RE = Sm and Nd, both show behavior similar to the Ce compounds on the Fe-rich side. 14,20,21 However, on the Coend, namely, SmCoAsO and NdCoAsO, complex magnetic properties were observed, in which the Co moments sequentially undergo an FM and an AFM transition, followed by an AFM transition of 4f electrons at T Sm N = 5 K and T Nd N = 3.5 K. 22,23 By applying a magnetic field, such an FM-AFM transition quickly shifts to lower temperatures, showing a large magnetoresistance effect up to the Curie temperature. 24 However, in GdCoAsO, the magnetic susceptibility exhibits a broad maximum at temperatures below T Co C , being different from the FM-AFM transition in the Sm-and Nd-case, and its origin remains unclear.…”

“…16,17 On the Co-rich side, the ferromagnetically ordered Co ions have a strong polarization effect on the AFM order of Ce moments. 14,18 On the other hand, in the Gd(Fe,Co)AsO series, which correspond to the case of a large 4f -magnetic moment, Fe/Co substitution also leads to a superconducting dome near the 3d-electron magnetic instability. 14 On the Co-rich side, in contrast to the Ce-case, the large moment of Gd ions is robust against the FM order of Co moments, and maintains its AFM order with a nearly unchanged Néel temperature over the entire doping range.…”

“…24 However, in GdCoAsO, the magnetic susceptibility exhibits a broad maximum at temperatures below T Co C , being different from the FM-AFM transition in the Sm-and Nd-case, and its origin remains unclear. 14,19,24 Since the transitions of the Co 3d electrons are susceptible to the application of magnetic field in these compounds, e.g., SmCoAsO and NdCoAsO, 24 it is also interesting to tune the d-f magnetic coupling in GdCoAsO and investigate the associated phenomena.…”

“…9,10 Elemental substitutions in CeFeAsO, e.g., Fe/Co or As/P, may induce superconductivity while suppressing the magnetic order of Fe-3d electrons. [11][12][13][14][15] On the other hand, CeFePO is a paramagnetic (PM) heavy fermion metal. 1 Evidence for a magnetic quantum phase transition was shown in CeFeAs 1−x P x O, where the AFM state is separated from the PM heavy fermion state, and a ferromagnetic order develops in the intermediate doping region.…”